1. Field of the Invention
The invention relates to the field of magnetoresistive detectors for detecting single wall domains.
2. Prior Art
Numerous detectors are well-known for detecting single wall magnetic domains (bubbles) which are moved in a layer of magnetic material by an in-plane rotating magnetic field. Most commonly these detectors rely upon the magnetoresistive properties of the permalloy materials used in magnetic bubble devices. The magnetic bubble is first expanded and then magnetically coupled to a magnetoresistive element and the resistance change of this element sensed. An expanded bubble or a plurality of bubbles provides a sufficient resistance change in the element to permit reliable sensing. Examples of detectors which employ an expanded bubble and a plurality of bubbles are described in U.S. Pat. Nos. 3,781,832 and 3,858,189.
One of the more commonly used detectors includes a plurality of columns of transfer elements such as chevron elements to first expand the bubble before detection. After expansion the bubble is propagated to interlinked chevron elements fabricated from a permalloy material having a magnetoresistive property. This type of detector, including an improvement in the shape of the chevron elements, is described in U.S. Pat. No. 4,019,177.
In magnetic bubble memories, it is common to merge two independent streams of bubbles into a single stream or train of bubbles before detection. In each stream the bubbles are spaced-apart and out of phase with the bubbles in the other stream so that the merging occurs without interfering with the data stored in each stream. The singlestream of data is coupled to a detector which includes an expander and the interlinked chevrons which form the detector stage. These detectors most frequently include an additional column of interlinked chevrons used as a "dummy" detector. The dummy detector is electrically coupled to the detector stage to provide noise cancellation, for example, to remove the effects of the in-plane rotating field. Two terminals of the detector stage and dummy stage are coupled to an off-chip sensor which includes a pair of external resistors forming a bridge circuit with the detector stage and dummy stage.
One advantage to the above-described detection apparatus is that only two terminals (package pins) are required for sensing the data in the two independent (merged) data streams. However, this detection means has the disadvantage of having the bubbles relatively close together (after merging) as they pass through the detector. This proximity of the bubbles to one another results in less reliable detection in the detector.
As will be seen, the present invention provides a magnetoresistive detection apparatus wherein the data in two independent streams of bubbles are sensed at two terminals and thus only two pins on the memory package are required. However, unlike the prior art detection scheme, the bubbles are not merged before detection and thus the problem associated with the close bubbles is eliminated.